1. Field of the Invention
The present invention relates to an apparatus and a method for controlling a four-wheel drive vehicle, which can implement optimum controls even when an on-board traction control device operates so that driving force transmitted to a road surface is limited.
2. Description of the Related Art
In recent years, a traction control device is equipped on automotive vehicles which controls engine outputs and braking forces so as to prevent driving wheels from slipping in order to thereby secure acceleration, steering stability and vehicle behavior stability when the automotive vehicle starts from stand still conditions.
In a case where the traction control device like this is equipped on a four-wheel drive vehicle, there may occur unfavorable conditions to the vehicle in which a vehicle behavior control device interferes with controls by a power distribution control device equipped on the four-wheel drive vehicle. For eliminating these problems, Japanese Patent Unexamined Publication No. Sho. 61-37541 discloses the four-wheel drive vehicle in which a four-wheel drive is forcibly shifted to a two-wheel drive when a slip control operation is activated.
On the other hand, Japanese Patent Unexamined Publication No. Hei. 1-266049 discloses a technology in which a limited slip differential function is stopped to provide for a so-called locked-up four-wheel drive when there occurs an acceleration slip. Namely, the acceleration slip suppression effect results from stopping the limited slip differential function.
However, with the former prior art, the four-wheel drive is shifted to the two-wheel drive at the same time as the slip control operation is activated. This may lead to a possibility that the drivability of the vehicle is remarkably deteriorated. In addition, there is caused a problem that the driver is forced to feel large physical disorders when there occurs a drastic change in vehicle characteristics that would result when the four-wheel drive is shifted to the two-wheel drive.
In addition, with the latter prior art, slips on the four wheels are rather promoted, leading to a risk that a sufficient traction performance cannot be secured.
The present invention was made in view of these situations. An object of the present invention is to provide an apparatus and a method for controlling a four-wheel drive vehicle which can continue to implement optimal limited slip differential controls without unnecessarily interfering with the traction control to maintain a superior drivability without making the driver feel the physical disorder, and to ensure that the stability of the vehicle can be improved by properly controlling slips on the four wheels.
With a view to attaining the object, according to a first aspect of the invention, there is provide a control apparatus for a four-wheel drive vehicle comprising:
a traction control unit for controlling a driving force when there occurs a wheel slip on a wheel;
a transfer device with a limited slip differential unit for limiting a differential rotational action between front and rear wheels for distributing said driving force transmitted thereto between the front and rear wheel sides; and
a limited slip differential control unit for setting a limited slip differential force by the limited slip differential unit,
wherein the limited slip differential control unit compensates for the limited slip differential force by said limited slip differential control unit in such a manner that the limited slip differential force is increased when there occurs a wheel slip exceeding a predetermined value at least either one of front left and right wheels or rear left and right wheels.
According to a second aspect of the invention, there is provided the control apparatus for the four-wheel drive vehicle as set forth in the first aspect of the invention, wherein the limited slip differential control unit sets the limited slip differential force according to at least either one of the driving force inputted into the transfer device or the difference in the number of rotations between the front and rear wheels.
According to a third aspect of the invention, there is provided the control apparatus for the four-wheel drive vehicle as set forth in the first or second aspect of the invention, wherein the limited slip differential control unit implements the increasing compensation of the limited slip differential force at the time of running at a low speed.
According to a fourth aspect of the invention, there is provided the control apparatus for the four-wheel drive vehicle as set forth in any of the first to third aspects of the invention, wherein the limited slip differential control unit implements the increasing compensation of the limited slip differential force according to at least a steering angle.
According to a fifth aspect of the invention, there is provided the control apparatus for the four-wheel drive vehicle as set forth in any of the first to fourth aspects of the invention, wherein the limited slip differential control unit implements the increasing compensation of the limited slip differential force according to at least a vehicle speed.
According to a sixth aspect of the invention, there is provided the control apparatus for the four-wheel drive vehicle as set forth in the fifth aspect of the invention, wherein the increasing compensation according to the vehicle speed is implemented when the steering angle is small.
According to a seventh aspect of the invention, there is provided the control apparatus for the four-wheel drive vehicle as set forth in any of the first to sixth aspects of the invention, wherein the traction control unit operates so that the driving force transmitted to a road surface is limited only when there occurs a wheel slip exceeding the predetermined value at least either one of the front left and right wheels or the rear left and right wheels.
Namely, with the control apparatus for the four-wheel drive vehicle according to the first aspect of the invention, the limited slip differential unit limits the differential rotation between the front and rear wheels with the limited slip differential force set by the limited slip differential control unit, and the transfer device distributes the driving force from the power source between the front and rear wheel for driving the vehicle. In addition, the traction control unit controls the driving force when there occurs a wheel slip on the wheel. Here, the limited slip differential control unit compensates for the limited slip differential force that is to be set by the limited slip differential control unit such that the limited slip differential force is increased when there occurs the wheel slip exceeding the predetermined value at least either one of the front left and right wheels or the rear left and right wheels. Consequently, in case there occurs the wheel slip exceeding the predetermined value at least either one of the front left and right wheels or the rear left and right wheels (for example, the wheel slip that occurs due to the difference in friction coefficient between the left and right wheels of the vehicle and the road surface, respectively, in normal driving conditions), the traction control unit is activated to control to suppress the driving force transmitted to the road surface, and the limited slip differential control unit is activated to increase the limited slip differential force for limiting the differential action between the front and rear wheels. Therefore, stable driving can be provided. Thus, the limited slip differential control unit operates properly when there occurs the wheel slip either one of on the left or right wheels and continues to implement the optimal limited slip differential control without necessarily interfering with the traction control. Therefore, it is possible to maintain the superior drivability without making the driver feel the physical disorder. In addition, the limited slip differential control unit can properly suppress wheel slips on the four wheels together with the traction control unit to thereby ensure the improvement in vehicle stability.
According to the second aspect of the invention, there is provided the control apparatus for a four-wheel drive vehicle as set forth in the first aspect of the invention, wherein the limited slip differential control unit sets the limited slip differential force according to at least either one of the driving force inputted into the transfer device or the difference in the number of rotations between the front and rear wheels. Therefore, the limited slip differential force is properly set in accordance with the driving conditions of the vehicle, thereby making it possible to ensure that the driving stability of the vehicle is improved.
According to the third aspect of the invention, there is provided the control apparatus for a four-wheel drive vehicle as set forth in the first or second aspect of the invention, wherein the limited slip differential control unit implements the increasing compensation of the limited slip differential force to be set by the limited slip differential control unit at the time of running at low speeds which is predetermined. Namely, in a case where the vehicle speed is fast, since the input torque is small and there occurs a little slip, no compensation is implemented for driving in those conditions to thereby improve the accuracy.
According to the fourth aspect of the invention, there is provided the control apparatus for the four-wheel drive vehicle as set forth in any of the first to third aspects of the invention, wherein the limited slip differential control unit implements the increasing compensation of the limited slip differential force to be set by the limited slip differential control unit according to at least a steering angle. Namely, in a case where the driver steers largely, if the increased compensation of the limited slip differential force is implemented, the vehicle characteristics are drastically shifted to the locked up condition of the four-wheel drive. This leads to a possibility that the driver feels a physical disorder. To cope with this, for example, it is designed such that the larger the steering angle becomes, the less the increasing compensation of the limited slip differential force is implemented. Therefore, the physical disorder can be reduced.
According to the fifth aspect of the invention, there is provided the control apparatus for a four-wheel drive vehicle as set forth in any of the first to fourth aspects of the invention, wherein the limited slip differential control unit implements the increasing compensation of the limited slip differential force to be set by the limited slip differential control unit according to at least the vehicle speed. Namely, since the faster the vehicle speed becomes, the smaller the input torque becomes and hence there occurs less wheel slip. For example, it is designed such that the faster the vehicle speed becomes, the less the increasing compensation of the limited slip differential force is implemented. Therefore, the control accuracy is improved.
According to the sixth aspect of the invention, there is provided the control apparatus for the four-wheel drive vehicle as set forth in the fifth aspect of the present invention, wherein the increasing compensation implemented by the limited slip differential control unit corresponding to the vehicle speed is implemented when the steering angle is small. In a case where the increasing compensation is implemented corresponding to the vehicle speed, in consideration of the effect of the steering angle (the occurrence of the physical disorder felt by the driver when he or she steers the steering wheel), as described above, the increasing compensation is implemented in such conditions that the steering angle is small and that the vehicle characteristics are little affected even if the vehicle characteristics are shifted to the permanent four-wheel drive.
According to the seventh aspect of the invention, there is provided the control apparatus for the four-wheel drive vehicle as set forth in any of the first to sixth aspects of the invention, wherein the traction control unit operates so that driving force transmitted to the road surface is limited only when there occurs the wheel slip exceeding the predetermined value at least either one of the front left and right wheels or the rear left and right wheels. Therefore, it is ensured that unnecessary torque is suppressed relative to the wheel slip occurring either on the left or right wheels, the vehicle stability being thereby improved.